This invention relates generally to imaging systems, and more particularly, to nuclear medicine imaging systems having pixelated detectors.
Nuclear medicine imaging systems, for example, Single Photon Emission Computed Tomography (SPECT) and Positron Emission Tomography (PET) imaging systems, use several image detectors, such as one, two or three detectors, to acquire imaging data, such as gamma ray or photon imaging data. The image detectors may be, for example, gamma cameras that acquire two-dimensional views of three-dimensional distributions of radionuclides emitted from an object (e.g., a patient) being imaged. The image detectors may be rotated about a patient to acquire a plurality of two-dimensional images (also referred to as projections) to create a multi-dimensional image of a structure of interest or photons transmitted through the object. A non rotating plurality of detectors may also be used to acquire the plurality of planar images as well. The rotating nuclear medicine systems are referred to as single photon emission computed tomography (SPECT) imaging systems. In SPECT systems, 40, 60 or more projections may be acquired, which are then reconstructed to generate a three-dimensional dataset. Backprojection and iterative reconstruction algorithms known in the art may then use information about the physical construction and properties of the imaging system to reconstruct the dataset into three-dimensional and/or four-dimensional representations. The three-dimensional or four-dimensional dataset then may be used to show different slices along or regions within the dataset and display the results as an image similar to images obtained from other tomographic imaging scans, such as, magnetic resonance imaging (MRI) and computed-tomography (CT) scans.
Gamma cameras for detecting photons for SPECT, PET, etc. may be fabricated from semiconductor materials, such as cadmium zinc telluride (CdZnTe), often referred to as CZT, cadmium telluride (CdTe), gallium arsenide (GaAs) and silicon (Si), among others. These semiconductor gamma cameras or semiconductor radiation detectors typically include arrays of pixelated detectors or detector modules. The response of these semiconductor radiation detectors to radiation is a localized current pulse that is detected by localized electronic circuits. These localized electronic circuits are often provided as Application Specific Integrated Circuits (ASICs) attached to and combined with the semiconductor radiation detectors in an imaging module.
These semiconductor radiation detectors suffer from different problems. In particular, one problem is the variations in both the semiconductor radiation detectors and the ASIC response from position to position in the imaging plane. The response constitutes both the size of the electrical pulse for radiation of a given energy, as well as the probability of giving a response of that size. The variations degrade image quality by giving an uneven background in the image against which physicians attempt to detect subtle features. This image degradation makes it difficult, sometimes impossible, to detect these subtle features. Further, these variations can also constitute a lack of energy resolution for the incident radiation and prevent applications where two isotopes are measured at the same time, with one isotope causing energy down-scatter in the spectrum and overlapping the energy region of the second isotope. Another problem with these semiconductor radiation detectors or attached electronics (e.g., ASIC) is the non-linearity (non-proportionality) in the response to different energies of incident flux. Still another problem with these semiconductor radiation detectors is the unresponsiveness of various pixels due to different causes, which results in pixels that are not operating properly.
Thus, in semiconductor radiation detectors, such as pixelated semiconductor gamma cameras, problems exist as a result of the response variation on individual crystal domains and different instances of the electronics readout, the non-linear response to the energy of the incident radiation flux, and slow variations of these parameters in time.